Flameproof moisture-resistant coatings for electrical components

ABSTRACT

ELECTRICAL COMPONENTS HAVING PROTECTIVE COATINGS WHICH ARE BOTH FLAMEPROOF AND HIGHLY MOISTURE-RESISTANCE, AND COMPOSITIONS FOR PRODUCING THOSE COATINGS, ARE DESCRIBED. THE COATINGS CONSIST ESSENTIALLY OF MONOMETHYL POLYSILOXANE RESIN TOGETHER WITH SELECTED REFRACTORY FILLERS AND SUSPENSION AGENTS. OPTIONAL MINOR CONSTITUENTS INCLUDE OTHER SILICONE RESINS, TETRAALKYL ORTHOSILICATES AND PIGMENTS. THE FLAMEPROOF AND MOISTURE-RESISTANT QUALITIES OF THE COATINGS ARE DUE TO THEIR UNIQUE COMBINATION OF REFRACTORY, HYDROPHOBIC AND POROMERIC PROPERTIES.

United States Patent O 3,834,938 FLAMEPROOF MOISTURE-RESISTANT COATllNGS FOR ELECTRICAL COMPONENTS Lawrence G. Boclrstie, Jr., Bradford, Pa., assignor to Corning Glass Works, Corning, N.Y. No Drawing. Continuation-impart of abandoned application Ser. No. 75,695, Sept. 25, 1970. This application May 26, 1972, Ser. No. 257,325

Int. Cl. C0911 3/28 US. Cl. 117-201 9 Claims ABSTRACT OF THE DISCLOSURE Electrical components having protective coatings which are both flameproof and highly moisture-resistant, and compositions for producing those coatings, are described. The coatings consist essentially of a monomethyl polysiloxane resin together with selected refractory fillers and suspension agents. Optional minor constituents include other silicone resins, tetraalkyl orthosilicates and pigments. The flameproof and moisture-resistant qualities of the coatings are due to their unique combination of refractory, hydrophobic and poromeric properties.

CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 75,695, filed Sept. 25, 1970, now abandoned.

BACKGROUND OF THE INVENTION The present invention has general utility in the manufacture of electrical components, and particular application in the manufacture of oxide film resistors, so called because the resistive element thereof is usually a thin metal-oxide film. Such resistors typically comprise a dielectric substrate such as a glass and an electrically-couductive resistive layer disposed on the surface of the substrate such as a doped tin oxide film, to which electrical contact is typically made with suitable terminal electrodes and lead wires. One of the problems of the art is the protection of the oxide film resistive element from moisture. Moisture in the resistor acts as an electrolyte during use, reacting with the metal-oxide film and permanently altering its resistive value. Accordingly, protective coatings for such resistors should be completely moisture resistant.

A further requirement of such protective coatings is that they be flameproof and arc resistant. Overloaded resistors, for example, often reach temperatures of 400600 C. in spots before burning out, and may also are prior to failure, possibly damaging other circuit elements. It is desirable in such circumstances that the resistor coating be of a fiameproof, insulating material, so that resistor failure will occur in a self-contained, fuse-like manner. Similarly, a fiameproof coating is useful to contain flaming and areing in other electrical components which might be subject to overload.

The most effective prior art attempts to achieve a flameproof resistor coating have involved the use of compositions containing siliceous liquid vehicles or binders such as the aqueous alkali silicates and the tetraalkyl orthosilicates. These materials provide good resistance to flaming in overloaded resistors but exhibit poor moisture resistance, thus necessitating the use of a further waterproof coating when both flame-proofing and moisture resistance are required. Common water-proof coatings, on the other hand, being generally organic compounds or mixtures thereof, do not demonstrate adequate fiameproof characteristics for many electronic applications.

Resistor coating systems offering a combination of flameproof and moisture-resistant properties are known. United States Pat. No. 3,562,007, for example, describes a three- "ice component resistor coating system which is both moistureresistant and riameproof. However, the need for three separate and distinct coating compositions adds significantly to the cost of the resistor. What is therefore required is a single-component coating which can provide both fiameproof and moisture-resistant properties to electronic circuit elements.

SUMMARY OF THE INVENTION The present invention comprises compositions suitable for coating electrical components such as resistors, capacitors, inductors and the like, consisting essentially of a monomethyl polysiloxane resin, refractory oxide fillers, suspension agents, and organic solvents. Coatings produced from these compositions, being composed of the monomethyl polysiloxane resin in combination with specified quantities of refractory oxide fillers and suspension agents, are both completely flameproof and highly moistureresistant under the adverse conditions of high humidity and high temperature. Non flammability to temperatures of up to about 1000 C. has been observed with these coatings. This combination of proportions is attributable to the poromeric nature of the coating and to the unexpected flame-resistance of the cured resin-filler-extender system. Further advantages include good chip resistance in the coating and complete compatibility thereof with resistive films, as well as extended pot life of the compositions in the manufacturing process.

The invention further comprises an electrical component such as a resistor, capacitor, inductor, or other electrical circuit element or combination thereof, having a flameproof moisture-resistant coating thereon consisting essentially of a monomethyl polysiloxane resin in combination with specified quantities of refractory oxide fillers and suspension agents.

The invention further comprises a method of making a flameproof, moisture-resistant electrical component comprising the steps of mixing a coating composition consisting essentially, in weight percent, of about 4-24% of a monomethyl polysiloxane resin, l070% of refractory oxide fillers, 220% of suspension agents, and 10-50% of organic solvents, applying the coating composition to an electrical component, and curing the composition to volatilize the organic solvent and polymerize the monomethyl polysiloxane resin, thereby forming a poromeric yet highly moisture-resistant flameproof coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specifically, my invention includes new compositions for coating electrical circuit components, particularly thinfilm electrical resistors, consisting essentially, in weight percent, of about 424% of a monomethyl polysiloxane resin, about 1070% of at least one refractory oxide filler, about 2-20% of at least one suspension agent, and about 10-50% of organic solvents. Optional constituents include 0l5% of pigments and not more than a total of about 10% by weight of other silicone resins and specified alkyl silicates. The optional silicone resins preferably have a high SiO content in the cured state (at least about 60%) and form highly crosslinked polymers upon curing.

Aluminum oxide (alumina) and silicon dioxide (silica) are examples of refractory oxide filler suitable for the coating compositions of the present invention. My preferred composition contains both silica and alumina, with silica being present in amounts ranging from about 10- 40% and alumina being present in amounts ranging from about 10-30% by Weight. Other suitable refractory oxide fillers such as zirconium oxide, zircon, MgO, Ta O W0 M00 SiC, spinel and forsterite may also be used if desired.

The mesh size of the aluminum oxide to be used in these compisitions is important, since the texture of the coatings will be affected if excessively large particles are present. Tabular alumina 325 mesh gives very satisfactory results in all compositions although mesh sizes up to and including about 200 mesh have also been employed.

The mesh size of the silicon dioxode used in the composition is also important. If excessively fine silica is used, the coating will occasionally crack during curing, while excessively large particle sizes result in an extremely rough and abrasive coating. A good grade of silicon dioxide powder ranging from about 150250 mesh will give satisfactory results, with about 200 mesh powdered fused silica being preferred.

Mica and aluminum silicate powder are examples of materials which serve as both extenders and suspension agents. My preferred composition contains both mica and aluminum silicate powder, with mica being present in amounts ranging from about 210% and aluminum silicate being present in amounts ranging from about 2l0%. The mica preferred for my composition is white, waterground mica, 325 mesh. However, 160 and 1000 mesh mica were also used, and forms of mica other than white, water-ground may be used if desired. The aluminum silicate powder should preferably have a particle size of about 325 mesh or smaller. Of course, other well-known suspension agents such as clays and diatomaceous earths may be substituted for mica and aluminum silicate if desired.

Titanium dioxide, cobaltous aluminate, iron oxide, and chromium oxide are examples of optional pigments useful in coloring the coatings of the invention, and they may be present in amounts from 045% by weight of the composition. Other pigments and dyes may also be used depending upon the color desired and upon the temperature requirements of the coating. Thus, organic dyes are suitable where heat resistant colors are not required.

While the above fillers and suspension agents have been used in prior art resistor coating compositions, they have not previously been employed with monomethyl polysiloxane resins, and it has been found that the aboverecited proportions of fillers and suspension agents must be strictly adhered to if a flameproof moisture-resistant coating is to be produced. These proportions are particularly important with regard to the poromeric quality of the coating, which must be retained if adequate moisture resistance is to be realized. The monomethyl polysiloxane resins employed herein do not form a poromeric coating in the absence of the disclosed additional constituents in the proper porportions, and are thus unsuitable for use as a sole coating material for electrical components.

The flameproof quality of the coatings of the invention also depends on the use of the specified components in the proportions described. Monomethyl polysiloxane resins, like other silicones, are somewhat heat-resistant, but they are not flameproof under the conditions of severe resistor overload. Surprisingly, however, the coatings of the present invention are completely fiameproof at temperatures in excess of those encountered during resistor overload notwithstanding the presence of organic constitutents therein.

As is well known in the art, it is generally the resin system of a coating which limits the flameproof and moistureand arc-resistant qualities thereof, and a primary problem of the art in this area has been to find a flameproof material which additionally provides both moisture and are resistance. The problem is illustrated by the tetraethyl orthosilicate-based coatings commonly used in the prior art to protect film resistors. Coatings containing this material are cured by causing the tetraethyl orthosilicate vehicle to undergo a hydrolysis reaction as follows:

The enthanol any any excess water are driven off during the curing process to leave an inorganic siliceous product which is highly resistant to burning, but quite porous and susceptible to moisture penetration. Consequently, according to the prior art, a further waterproof coating is commonly used with this material to provide an acceptable level of moisture resistance.

Materials used in the prior art as water resistant coatings include the silicones or organopolysiloxanes, since these materials are known to have good moisture resistant qualities and higher resistance to thermal decomposition than other organic resins. However, according to the prior art, the use of these polymeric materials has been limited to applications wherein a flameproof characteristic is not a requirement, because the readily-oxidizable organic groups in the cured polymers invariably reduce the flameproof qualities of the composite coating below acceptable levels.

There have been numerous attempts to impart improved flameand heat-resistance to silicone materials, as by adding flame-retardant compounds or refractory oxide fillers thereto. However, in no case known to the inventor have such additives increased the flame-resistance of these materials to the point where they are sufiiciently flameproof and arc-resistant for use in single-component resistor coating systems. Also, many commonly-used flameretardant compounds are not compatible with thinfilm metal oxide resistive elements, causing unacceptable variations in the resistive value thereof.

I have now discovered that monomethyl polysiloxane resins having a methyl-silicon ratio of about 1:1, when used as the primary resin in the disclosed refractory oxide coating system, form a poromeric coating which is flameproof nothwithstanding the presence of organic groups therein. These monomethyl polysiloxane resins are partially polymerized in a crosslinked fashion in the uncured state, having a molecular weight range of about 1000-3000. The monomer from which they are made has three functional alkoxy groups, in addition to one methyl group, attached to each silicon atom, as follows:

During polymerization of the resin, each of the functional groups is readily displaced from the molecule by hydrolysis to provide a site for bonding with other sites through condensation, thus allowing three-dimensional linking and the formation of a highly crosslinked, highly siliceous polymerization product. The methyl groups, on the other hand, are not easily displaced and remain bonded to the silicon atoms as part of the cured coating.

When completely cured, these polymers have an unusually high silica content of about 68 to 88 percent by weight. This high silica content contributes to the flameproof quality of the resultant coating. The cured polymer also contains methyl groups with which give a high degree of moisture resistance without affecting the flameproof properties of the coating.

Although the character of the alkoxy groups present in the methyl polysiloxane is not critical for purposes of the present invention, since the compounds formed therefrom are driven off during the polymerization process, it is preferred that the organic group retained in the cured coating after polymerization be the methyl group, ethyl group, propyl group, or, optionally, the phenyl group. If other long chain groups are retained, as for example the hexadecyl group, the resulting coating will be water resistant, highly siliceous and highly crosslinking, but will not be sufiiciently flameproof for high temperature resistor applications. The methyl polysiloxanes are particularly de sirable in the production of flameproof coatings, but the phenyl polysiloxanes and the methyl-phenyl copolymers will also yield coatings which are useful for their flame-resistant qualities.

The monomethyl polysiloxane resin preferred for the purposes of the present invention is prepared from methyl trialkoxysilane and is in a state of partial polymerization as introduced to the coating batch, having a threedimensional (non-linear) polymeric structure and a molecular weight ranging from about 15002000. It is a solid material containing hydroxyl and alkoxy groups which is soluble in polar solvents such as acetone, isopropanol, etc. some non-polar solvents such as benzene, toluene, etc., and po'lar-non-polar solvent mixtures. It flows readily when heated and is compatible with most fillers, extenders, colorants, and dyes. It is a thermosetting resin which, upon heating to temperatures between about 150250 C., is rapidly cured to a chemically inert, transparent, glassy material having a three-dimensional polymeric structure and containing about 80% silicon and oxygen by Weight.

Examples of suitable monomethyl polys'iloxane resins are shown in United States Pats. Nos. 3,389,114 and 3,460,980. These patents also describe the preparation of flexible, transparent, coatings from monomethyl polysiloxane resins; however such coatings are not inherently flameproof under the conditions of severe resistor overload, nor are they poromeric so as to provide a suitably moisture-resistant coating for electrical circuit elements. Thus, the amount of monomethyl polysiloxane resin incorporated in the coating composition must be carefully maintained within about 424% by weight, corresponding to about 635% by weight of the cured poromeric coating, in order to obtain the desired combination of properties.

I have found that certain specified alkyl silicates may also be included in the composition as optional constituents of the vehicle or resin system without loss of the moisture resistant quality of the coating. Such silicates include tetraalkyl orthosilicates, either unhydrolyzed or in varying degrees of prehydrolysis (ranging from about -90%), condensed tetraalkyl orthosilicates, condensed or prepolymerized ethyl silicates, and mixtures of the above. An example of a preferred silicate is a mixture composed of ethyl polysilicates with an average of five silicon atoms per molecule, with the silica content for the mixture being about 40% by weight. Such material is commonly known as ethyl silicate 40, and is a polymerized form of tetraethyl orthosilicate which is commercially available. It is a water-white liquid having a specific gravity in the range from about 1.055 to about 1.065 and a viscosity of about 3.9 centipoises at 20 C.

I have also found that other silicone resins may be used as part of the vehicle system in these compositions as long as they are highly crosslinking and highly siliceous (at least about 60% silica after curing) so as to be thermally and dimensionally stable upon curing. One such resin is sold by Dow-Corning Corporation as Dow-Corning 2103 Laminating Resin. This material is an organopolysiloxane resin in a toluene solvent having a solids content of about 60% which is characterized by the manufacturer as a high-pressure silicone laminating resin having excellent heat stability and low water absorption. Although the manufacturer suggests the use of a catalyst such as triethanol amine for proper curing of this material, I have found that, when used in the compositions of the present invention, neither elevated pressures nor catalysts are required.

The addition of such silicone resins to the composition of my invention is desirable to decrease the porosity of the resulting coating, as well as the cost of the batch materials. However, such additions do not contribute to the moisture resistance of the coatings and moreover tend to increase the flammability thereof, if used beyond specified proportions. Consequently, they should not exceed the amounts specified, nor should the weight ratio of secondary silicone resins and other specified silicates to monomethyl polysiloxane resin exceed about 12.

Solvents which are suitable for controlling the viscosity of the coating composition according to the present invention include ethylene glycols such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, and 5 ethylene glycol monobutyl ether. The choice of a solvent is somewhat important inasmuch as the uncured resins should be completely dissolved and/ or dispersed therein to insure thorough mixing prior to the application of the coating. Ethyl alcohol is an optional solvent which can comprise up to about 50% by weight of the total solvent used in the composition, but greater amounts are not recommended because the solubility of the optional silicone laminating resins therein may be limited. Of course, any other organic solvents which are compatible with the silicone resins and the specified silicates used herein may also be employed.

A coating composition within the aforementioned ranges may be prepared by conventional methods of mixing and blending; the particular method employed has relatively little effect on the properties of the cured coating. However, it is found that thorough blending of the coating materials is best obtained if the resinous constituents are dispersed in the solvents prior to the addition of the other batch materials. The pot life of the uncured compositions of the present invention is significantly increased over that of many prior art compositions such as, for example, the tetraethyl orthosilicate compositions. In addition, the coatings of the present invention are found to be more compatible with thin-film resistive elements than prior art coatings, in that they do not significantly affect resistive values during processing and testing. Prior art coatings caused drifts in resistor values of up to about 5% as a result of interaction with the resistive film during the manufacturing process.

The method of applying the coating composition likewise has little effect on the qualities of the cured coating. The amount of solvent used can be varied to obtain the desired viscosity and the material may then be applied by conventional methods such as spraying, dipping, rolling, and the like.

The following numbered examples illustrate some specific coating compositions and suggested ranges, expressed in parts by weight, suitable for producing coatings exhibiting the desirable combination of flameproof and water resistant properties. It is to be understood that these examples are not limiting as to all applications suitable for the compositions disclosed therein, rather merely illustrative of compositions to achieve the particular end properties desirable for certain resistor applications. Obviously, adjustments may be made to obtain diiferent properties or the same properties differing in degree or having different tolerence values. 7

Example I 94-20 Ethylene glycol monoethyl ether. 30-5 Monomethyl polysiloxane resin. 64' Titanium dioxide (rutile). 12-8. Chromium oxide.

Mica (325 mesh). Aluminum silicate (325 mesh). Aluminum oxide (325 mesh).

96-50 I- Silicon dioxide (200 mesh).

Example III 88-20 Ethylene glycol monoethyl ether.

24-5 Monomethyl polysiloxane resin.

. Silicone laminating resin (Dow-Corning 2103 Resin).

Titanium dioxide (rutile).

. Cobaltous aluminate.

Mica (325 mesh).

Aluminum silicate (325 mesh).

. Aluminum oxide (325 mesh).

. Silicon dioxide (200 mesh).

Example III Ethylene glycol monoethyl ether. Monomethyl polysiloxane resin. Ethyl silicate 40 (Union Carbide).

6-4 Titanium dioxide (rutile).

124 Chromium oxide.

-3 Mica (325 mesh).

104 Aluminum silicate (325 mesh).

64-30 Aluminum oxide (325 mesh).

96-50 Silicon dioxide (200 mesh).

Example IV 88-20" Ethylene glycol monoethyl ether.

24-5+ Monomethyl polysiloxane resin.

6-2" Prehydrolyzed tetraethyl orthosilicate (50% prohydrolyzed).

6-4. Titanium dioxide (rutile).

10a; Mica (325 mesh).

10 Aluminum silicate (325 mesh).

6 -30 Aluminum oxide (325 mesh).

96- Silicon dioxide (200 mesh).

I claim:

The coatings of my invention are preferably cured by heating, which has the effect of accelerating the resin polymerization process and insuring maximum crosslinking in the polymer to provide a more durable coating. I have found that the polymerization reaction is time and temperature dependent and that, for electrical component applications, subjection of the coated component to temperatures ranging between about l50250 C. for times ranging about 2-15 minutes, depending upon the size of the component and the thickness of the coating, will provide good curing. Of course, longer times at varying temperatures may be employed, but since they do not significantly add to the quality of the coating, they are not deemed of particular economic or practical importance.

The cured coatings of the present invention may be characterized as flameproof and moisture resistant, preferably consisting essentially, in weight percent, of about 6-35% of a monomethyl polysiloxane resin, -89% of refractory oxide fillers, and 329% of suspension agents. They may additionally contain up to about 15% of the specified optional silicone resins or tetraalkyl orthosilicates, and up to about 22% of inorganic pigments. Preferably, the refractory oxide fillers include 15-58% silica and 15- 44% alumina.

The described coatings are hydrophobic because of the presence of methyl groups in the cured resin component thereof, and are poromeric. Thus, they have excellent moisture resistance because they inhibit the migration of moisture into the coating while permitting the rapid escape of moisture therefrom during the operation of the electronic component. This feature is particularly critical in the case of oxide film resistors because it insures that no moisture will be trapped at the surface of the resistive film to act as an electrolyte during resistor operation. Since the coating is disposed directly on the dielectric substrate and the electrically-conductive resistive oxide film, any moisture trapped by the coating would cause unacceptable variations in the resistive value of the film over the period of use. The poromeric feature of the coatings of the present invention together with their outstanding compatibility with thin oxide film resistance elements make them eminently suitable for use in combination with oxide film resistors.

The following numbered examples illustrate the combination of moisture resistant and flameproof qualities exhibited by the coatings of the present invention, when applied to resistors and tested according to standard methods.

Example V Thirty tin oxide film resistors having a resistance of 150K ohms were coated by spraying with a composition consisting essentially, in parts by weight, of 48 parts silica (200 mesh), 32 parts alumina (325 mesh), 5 parts aluminum silicate (325 mesh), 5 parts white water-ground mica (325 mesh), 6 parts chromium oxide, 3 parts titanium dioxide, 6 parts ethyl silicate 40, 12 parts monomethyl polysiloxane resin, and 44 parts ethylene glycol monomethyl ether. The coatings were then cured by subjecting the coated resistors to a temperature of 200 C. for a total period of 15 minutes. The resistors were then subjected to five days of 100% humidity at 66 C., while carrying a trickle current. The maximum change in resistive value observed during treating and after treating was 0.18% and the minimum change was 0.00%. The control samples used in the same test which Were coated with a high-grade but flammable silicone paint, exhibited a maximum change in resistive value of 0.14% and a minimum change of 0.05%.

Example VI Thirty K ohm resistors were coated, cured and tested as in the foregoing example. The maximum change in resistive value observed during and after the test was 0.30% and the minimum change was 0.06%. The silicone-painted control samples of the same resistor value showed a maximum change of 0.61% and a minimum change of 0.35%.

Example VII A total of thirty 150K ohm and 90K ohm resistors, coated as in the foregoing examples, were subjected to overloads of times their rated power capacity. In all cases, resistor failure occurred in a fuse-like manner, opening the circuit Without flaming or arcing, and thereby relieving the circuit from the overload and from further damage. The control samples, which were coated with a standard silicone coating, all burned during the overload test. Some arced and some became so hot that the glass substrate of the resistor melted.

1. In combination with an electrical resistor of the type comprising a dielectric substrate and an electroconductive, resistive layer disposed on said substrate, the improvement which comprises a flameproof, moistureresistant coating disposed over said resistor composed, in weight percent, of about 635% of a monomethyl polysiloxane resin, 1589% of refractory oxide fillers, 329% of suspension agents, 0l5% total of vehicle constituents selected from the group consisting of alkyl silicates and organopolysiloxane resins having a silica content of at least about 60% by weight after curing, and 022% total of inorganic pigments.

2. A coated electrical device comprising (a) an electrical component, and

(b) a flameproof, moisture-resistant coating disposed over said component consisting essentially, in weight percent, of about 635% of a monomethyl polysiloxane resin, 1589% of refractory oxide fillers, 329% of suspension agents, 0l5% total of vehicle constituents selected from the group consisting of alkyl silicates and organopolysiloxane resins having a silica content of at least about 60% by weight after curing, and 022% total of inorganic pigments.

3. A coated electrical device according to claim 2 wherein the refractory oxide fillers are selected from the group consisting of silica and alumina, and wherein the suspension agents are selected from the group consisting of mica and aluminum silicate.

4. A coated electrical device according to claim 3 wherein the vehicle constituents are selected from the group consisting of tetraethyl orthosilicate, prehydrolyzed tetraethyl orthosilicate, condensed tetraethyl orthosilicate, prepolymerized ethyl silicate, and organopolysiloxane resins having a silica content of at least about 60% by weight after curing, the inorganic pigments are selected from the group consisting of titanium dioxide, cobaltous aluminate, and chromium oxide.

5. A coated electrical device according to claim 4 wherein the electrical component is an oxide film resistor.

6. A method of making a flameproof, moisture-resistant electrical device comprising the steps of:

(a) mixing a coating composition consisting essentially, in weight percent, of about 4-24% of a monomethyl polysiloxane resin, 1070% of refractory oxide fillers, 2-20% of suspension agents, 1050% of organic 9 10 solvents, up to about 10% of vehicle constituents ments are selected from the group consisting of titanium selected from the group consisting of alkyl silicates dioxide, cobaltous aluminate, and chromium oxide. and organopolysiloxane resins having a silicon con- 9. A method according to claim 8 wherein curing the tent of at least about 60% by weight after curing, coating composition comprises heating to a temperature and up to about 15% of inorganic pigments; 5 in the range from about 150250 C. (b) applying the coating composition to an electrical component; and References Cited (c) curing the coating composition to volatilize the or- UNITED STATES 1P ATENTS and paymeme the monomethyl poly 3,455,732 7/1969 Hathaway, Jr. 117-137 7. A method according to claim 6 wherein the refrac- 3562007 2/1971 Bockstie 117 137 tory oxide fillers are selected from the group consisting 3598617 8/1971 Bockstie 117 137 of silica and alumina, and the suspension agents are select- 5 i ed from the group consisting of mica and aluminum 0c silicate. 1

8. A method according to claim 7 wherein the alkyl CAMERON WEIFFENBACH Pnmary Exammer silicates are selected from the group consisting of tetra- Us CL X R ethyl orthosilicate, prehydrolyzed tetraethyl orthosilicate, 117.437 218 221 condensed tetraethyl orthosilicate, and prepolymerized tetraethyl orthosilicate, and wherein the inorganic pig- 0 

